(a) Field of the Invention
The present invention relates to composite nickel fine powder in which an oxide and/or a double oxide of a metal element are attached to the surface of metal nickel fine particles and a fatty acid is further supported on the surface of the nickel fine particles provided thereon with the oxide and/or double oxide attached thereto. More specifically, the present invention pertains to composite nickel fine powder, which is excellent in resistance to heat shrinkage, which has a high tap density, which permits the formation of a printed wiring having a high density after the powder is formed into a paste and then dried, which permits the formation of an electrode having a high density after firing and which has characteristic properties suitable for use as a material for forming a conductive paste and in particular as an internal electrode material for multilayer ceramic capacitors.
(b) Description of the Prior Art
The multilayer ceramic capacitor is in general produced by alternately putting ceramic dielectric materials and internal electrodes in layers, followed by pressing them to each other and firing the resulting assembly to thus integrate the foregoing elements. The internal electrode for such a multilayer ceramic capacitor is in general produced by forming metal fine powder as an internal electrode material into a paste, printing the paste on a ceramic substrate, putting a plurality of the printed substrates in layers, integrating them by pressing with heating and then heating and firing the resulting assembly in a reducing atmosphere. As such internal electrode materials, there have conventionally been used, for instance, palladium and platinum. However, there have recently been developed various techniques, which make use of base metals such as nickel in place of noble metals such as platinum and palladium and a great advance has been made in such techniques.
If metal nickel powder is used as the internal electrode material, however, it has a tendency of undergoing rapid heat-shrinkage at a temperature around 700.degree. C., although this tendency depends on the particle size of the nickel powder.
On the other hand, the firing temperature used when producing a multilayer ceramic capacitor may vary depending on the compounds constituting a ceramic dielectric material and when using, for instance, a ceramic dielectric material consisting of a perovskite-type double oxide such as BaTiO.sub.3 or SrTiO.sub.3, it is required to use a firing temperature of not less than 1200.degree. C. The nickel fine powder used for forming an electrode undergoes heat-shrinkage severer than those observed for dielectric materials at such a high temperature. For this reason, the multilayer ceramic capacitor undergo delamination, crack-formation and/or warpage due to the heat-strain generated between the laminated dielectric layer and nickel layer and this in turn results in the deterioration of the quality of the multilayer ceramic capacitor.
As a means for eliminating this drawback, it would be regarded, as important, that the temperature at which any rapid heat-shrinkage is initiated is shifted toward the higher temperature side to thus approach the heat-shrinkage curve of nickel fine powder for forming a paste used in the production of a multilayer ceramic capacitor to the heat shrinkage curve of the ceramic base material. To this end, there has been proposed a technique for applying a metal oxide or a double oxide onto the surface of individual nickel fine particles.
However, such a technique for applying a metal oxide or a double oxide onto the surface of nickel fine particles suffers from such a problem that the surface area of individual fine particles increases and that the tap density thereof is reduced. Accordingly, it would be quite difficult to achieve a printed wiring having a desired high density when forming a conductive paste and then drying the same.